Comparison of AVHRR Cloud and SEVIRI HRV Cloud RGBs
The AVHRR Cloud RGB is compared to the SEVIRI HRV Cloud RGB, which is the most spectrally similar product among the standard SEVIRI RGBs. Note that it would be possible to create an RGB even more spectrally similar to the AVHRR Cloud RGB by using SEVIRI channels VIS0.6, VIS0.8 and IR10.8, but that is not a 'standard RGB' recommended by EUMETSAT.
In the standard SEVIRI HRV Clouds RGB the HRV (High Resolution Visible) channel is used to benefit from its higher resolution. (HRV is a broadband channel covering the 0.4 -1.1 micrometer region.)
The recipes of AVHRR Cloud and SEVIRI HRV Cloud RGBs are summarized in Table 2. The channels for the red and green components are different. The ranges - where the images should be linearly stretched - are the same.
AVHRR Cloud RGB | ||||
---|---|---|---|---|
Color beam | Channel | Range | ||
Red | VIS0.63 | 0 | 100 | % |
Green | NIR0.87 | 0 | 100 | % |
Blue | IR10.8 inverted | 323 | 203 | K |
SEVIRI HRV Cloud RGB | ||||
---|---|---|---|---|
Color beam | Channel | Range | ||
Red | HRV | 0 | 100 | % |
Green | HRV | 0 | 100 | % |
Blue | IR10.8 inverted | 323 | 203 | K |
Table 2: Recipes of the AVHRR Cloud RGB and SEVIRI HRV Cloud RGB schemes
Figs. 8a, 8b and 9 show 2-panel images of AVHRR Cloud and the best-matching SEVIRI HRV Cloud RGB images. Their spatial resolution is comparable.
Clouds have a similar appearance in both RGBs. Both RGB types contain information on cloud optical thickness and cloud top temperature: thick, cold high-level clouds are white; thin high-level cirrus clouds are bluish; while medium- and low-level clouds are yellow.
Snow appears yellowish in both RGB types. However, the AVHRR Cloud RGB may contain some information on the snow's state. If the snow is melting, it has some red tones (see the Alps in Fig. 9).
The colors of snow-free land are different in AVHRR Cloud RGBs and SEVIRI HRV Cloud RGBs.
- In the AVHRR Cloud RGB the color of snow-free land reflects the green vegetation fraction ('chlorophyll content').
- The SEVIRI HRV Clouds RGB conveys no vegetation information.
Figure 8a: AVHRR Cloud RGB (up) and SEVIRI HRV Cloud RGB (bottom) on 25 August 2016 at 08:34 and 08:40 UTC
Figure 8b: AVHRR Cloud RGB (left) and SEVIRI HRV Cloud RGB (right) observed over Russia (northeast of Belarus) on 22 June 2016 at 07:19 and 07:25 UTC
Figure 9: METOP-A AVHRR Cloud RGB (left) and SEVIRI HRV Cloud RGB (right) images on 12 April 2016 at 09:16 and 09:10 UTC